Add an ||= optional-assignment operator for Sass.

lib/sass.rb

@@ -363,6 +363,39 @@ $LOAD_PATH << dir unless $LOAD_PATH.include?(dir)
 #   #main {
 #     content: string(Hello, "Hubert" Bean.) }
 #
+# === Optional Assignment
+#
+# You can assign Sass constants if they aren't already assigned
+# using the ||= assignment operator.
+# This means that if the constant has already been assigned to,
+# it won't be re-assigned,
+# but if it doesn't have a value yet,
+# it will be given one.
+# For example:
+#
+#   !content = "First content"
+#   !content ||= "Second content?"
+#
+#   #main
+#     content = content
+#
+# is compiled to:
+#
+#   #main {
+#     content: First content; }
+#
+# However,
+#
+#   !content ||= "Second content?"
+#
+#   #main
+#     content = content
+#
+# is compiled to:
+#
+#   #main {
+#     content: Second content?; }
+#
 # === Default Concatenation
 #
 # All those plusses and quotes for concatenating strings

lib/sass/constant.rb

@@ -31,7 +31,7 @@ module Sass
     }
 
     # The regular expression used to parse constants
-    MATCH = /^#{Regexp.escape(CONSTANT_CHAR.chr)}([^\s#{(SYMBOLS.keys + [ ?= ]).map {|c| Regexp.escape("#{c.chr}") }.join}]+)\s*=\s*(.+)/
+    MATCH = /^#{Regexp.escape(CONSTANT_CHAR.chr)}([^\s#{(SYMBOLS.keys + [ ?= ]).map {|c| Regexp.escape("#{c.chr}") }.join}]+)\s*((?:\|\|)?=)\s*(.+)/
     
     # First-order operations
     FIRST_ORDER = [:times, :div, :mod]

lib/sass/engine.rb

@@ -270,11 +270,18 @@ module Sass
     end
 
     def parse_constant(line)
-      name, value = line.scan(Sass::Constant::MATCH)[0]
+      name, op, value = line.scan(Sass::Constant::MATCH)[0]
       unless name && value
         raise SyntaxError.new("Invalid constant: \"#{line}\"", @line)
       end
-      @constants[name] = Sass::Constant.parse(value, @constants, @line)
+
+      constant = Sass::Constant.parse(value, @constants, @line)
+      if op == '||='
+        @constants[name] ||= constant
+      else
+        @constants[name] = constant
+      end
+
       :constant
     end
 

test/sass/engine_test.rb

@@ -208,6 +208,11 @@ END
   def test_cr_newline
     assert_equal("foo {\n  a: b;\n  c: d;\n  e: f; }\n", render("foo\r  a: b\r\n  c: d\n\r  e: f"))
   end
+
+  def test_or_eq
+    assert_equal("foo {\n  a: b; }\n", render("!foo = b\n!foo ||= c\nfoo\n  a = !foo"))
+    assert_equal("foo {\n  a: b; }\n", render("!foo ||= b\nfoo\n  a = !foo"))
+  end
   
   private